Conventional television delivery services have relied primarily upon cable delivery systems to supply a large number of television channels to consumers. A drawback to cable delivery systems is the high cost of the physical infrastructure. In particular, transmission cables must be routed to every household to which the television services are offered. Another drawback is the physical limitations on the number of channels which can be carried through the cable.
An alternative to cable delivery systems is the use of satellite receivers. Early satellite systems used a large dish antenna which was directed to one of several geosynchronous television relay satellites. Such systems arc expensive and the size of the dish required limits where the systems can be used.
More recently, direct broadcast televisions systems using small parabolic receiving dishes, i.e., on the order of three feet in diameter or less, have become popular. Coupled with improvements in receiving electronics and the use of dedicated satellite systems, these types of systems are inexpensive enough to compete with established cable delivery systems and are superior in many locations where a cable infrastructure has not been completed.
However, conventional satellite-based direct-broadcast television systems suffer from bandwidth limitations which force the content providers to limit the number of stations which are provided to the consumer. Although the limitations are not typically as severe as those in cable-based systems, market pressures are forcing content suppliers to offer more programs than can be carried on a single satellite. For example, while satellite television providers have been recently granted the right to carry local television signals, many systems do not have the excess capacity to include these additional signals. In addition, reliability considerations caution against using only a single satellite for delivery since a failure of the satellite will shut down the entire system until the fault is resolved.
In response to these factors, direct broadcast television systems which use multiple satellites are presently contemplated. In a particular system, a constellation of three geosynchronous satellites is planned, wherein the satellites are spaced at ten-degree intervals. The use of multiple satellites complicates the reception of the broadcast signals because the antenna can only be directed towards one satellite. Signals received from additional satellites will be reflected to points outside of the focal point of the antenna and distortions will be introduced. To compensate for this distortion, more sophisticated circuitry must be used, increasing the cost of the overall system.
Shaped antennas are often used for systems which transmit and/or receive from multiple points. These antennas have a reflecting surface which has been modified to improve performance in selected environments. A typical application for shaped antennas is on the broadcast antennas carried by communications satellites. Because the broadcast signal from these satellites must be received across a wide area, e.g., the continental United States, the antennas arc shaped to produce a broadcast beam that spans many degrees with an essentially uniform signal strength. Such antennas are typically modified spherical or torroidial antennas.
Although there are a variety of shaped antennas for use in different environments, the present designs are not particularly configured to receive three separate beams from three narrowly spaced geosynchronous satellites, such as satellites spaced at ten-degree intervals. While use of a conventional reflector configuration provides adequate results, the side beams are distorted and therefore have lower signal strength and increased noise susceptibility. One remedy is to increase the dish size. However, small dishes are preferred for use in mass-marketed direct television delivery systems.
Accordingly, it would be advantageous to provide a specially shaped antenna which provides improved reception from three geosynchronous satellites that are spaced at ten-degree intervals.
It would also be advantageous if such an antenna were relatively small, e.g., on the order of 2 to 3 feet in diameter, so as to be usable in the mass-consumer market.